1. Field of the Invention
The present invention relates to a relief valve including a main valve unit and a pilot valve unit.
2. Description of the Prior Art
In the past, this type of relief valve has been known in the art which includes a main valve unit 1 and a pilot valve unit 2 as shown in FIG. 8.
The main valve unit 1 includes a poppet-type main valve member 3 which is slidably fitted in a main valve chamber 1a and its poppet 3a is pressed against a valve seat 1d arranged between an inlet port 1b and an outlet port 1c by means of a spring 4.
On the other hand, the pilot valve unit 2 includes a seat member 5 fixedly mounted at one end within a pilot valve chamber 2a, and the seat member 5 includes a control chamber 5b having a valve seat 5a, a choke 5c having its one end communicated with the control chamber 5b and its other end communicated with a small-space chamber 2b and a through hole 5e communicating the control chamber 5b with an outer peripheral annular portion 5d.
In addition, disposed in the pilot valve chamber 2a is a poppet valve 7 which is pressed against the valve seat 5a by a spring 6 and the force of the spring 6 is adjusted by an adjusting screw 8 to set a pilot pressure.
Also, there are provided an internal pilot passage 9a communicating the inlet port 1b of the main valve unit 1 and the small-space chamber 2b of the pilot valve unit 2 and having a choke 9d located on the inlet port 1b side, a passage 9b communicating the main valve chamber 1a of the main valve unit 1 with the annular passage 5d of the pilot valve unit 2, and a passage 9c communicating the outlet port 1c of the main valve unit 1 with the pilot valve chamber 2a of the pilot valve unit 2.
Then, when the fluid pressure in the control chamber 5b of the pilot valve unit 2 is smaller than the force of the spring 6, the fluid pressures in the inlet port 1b, the control chamber 5 and the main valve chamber 1a of the main valve unit 1 all become equal to one another, so that the main valve member 3 is pressed against the valve seat 1d by the force of the spring 4 and a fluid pressure due to the pressure receiving area difference of the main valve member itself, thus causing no fluid flow to the outlet port 1c.
When the fluid pressure at the inlet port 1b is increased, the poppet valve 7 is moved to the right in the figure against the spring 6 and it is separated from the valve seat 5a, thereby causing a pilot flow flowing from the inlet port 1b to the outlet port 1c through the internal pilot passage 9a, the small-space chamber 2b, the choke 5c, the control chamber 5b, the pilot valve chamber 2a and the passage 9c.
As a result, a pressure difference is produced across each of the chokes 5c and 9c so that the fluid pressure in the control chamber 5b becomes lower than the fluid pressure at the inlet port 1b and fluid pressure in the control chamber 5b is directed to the main valve chamber 1a of the main valve unit 1 through the through hole 5e, the annular passage 5d and the passage 9b, thus causing a pressure difference between the fluids in the main valve chamber 1a and the inlet port 1b to separate the main valve member 3 from the valve seat 1d against the spring 4 and thereby discharge the fluid from the inlet port 1b to the outlet port 1c.
When this occurs, the fluid pressure at the inlet port 1b is decreased and the fluid pressure in the control chamber 5b is decreased. Thus, the poppet valve 7 is balanced with the force of the spring 6 and the main valve member 3 maintains the setting pressure by a certain flow rate owing to the balance among the fluid pressure at the inlet port 1b, the fluid pressure in the main valve chamber 1a and the force of the spring 4.